Radial variations of the spectral index, calculated between 1 and 3 mm, when a single inner planet of 20 M_Jup mass is embedded in the disks and when two planets are in the disk, but the outer planet (15 M_Jup) is injected 3 Myr after the inner planet (20 M_Jup). The vertical lines correspond to the positions of the pressure maxima at the outer edge of the gap(s) carved by the planet(s).